assertTrue(Thread.holdsLock(mutex));
      return delegate().navigableKeySet();
    }

    @Override
    public @Nullable Entry<K, V> pollFirstEntry() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate().pollFirstEntry();
    }

    @Override
    public @Nullable Entry<K, V> pollLastEntry() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate().pollLastEntry();

  }

  @Override
  public NavigableSet<K> navigableKeySet() {
    return delegate.navigableKeySet();
  }

  @Override
  public @Nullable Entry<K, V> pollFirstEntry() {
    return delegate.pollFirstEntry();
  }

  @Override
  public @Nullable Entry<K, V> pollLastEntry() {
    return delegate.pollLastEntry();
  }

  @Override
  public @Nullable V put(K key, V value) {

  }

  @Override
  public NavigableSet<K> navigableKeySet() {
    return delegate.navigableKeySet();
  }

  @Override
  public @Nullable Entry<K, V> pollFirstEntry() {
    return delegate.pollFirstEntry();
  }

  @Override
  public @Nullable Entry<K, V> pollLastEntry() {
    return delegate.pollLastEntry();
  }

  @Override
  public @Nullable V put(K key, V value) {

     * loop. Instead, we test standardLastEntry manually below.
     */

    @Override
    public @Nullable Entry<K, V> pollFirstEntry() {
      return standardPollFirstEntry();
    }

    @Override
    public @Nullable Entry<K, V> pollLastEntry() {
      return standardPollLastEntry();
    }

    @Override

    @Override
    public NavigableSet<K> navigableKeySet() {
      return fromMap().navigableKeySet();
    }

    @Override
    public @Nullable Entry<K, V2> pollFirstEntry() {
      return transformEntry(fromMap().pollFirstEntry());
    }

    @Override
    public @Nullable Entry<K, V2> pollLastEntry() {
      return transformEntry(fromMap().pollLastEntry());
    }

    @Override

    @Override
    public NavigableSet<K> navigableKeySet() {
      return fromMap().navigableKeySet();
    }

    @Override
    public @Nullable Entry<K, V2> pollFirstEntry() {
      return transformEntry(fromMap().pollFirstEntry());
    }

    @Override
    public @Nullable Entry<K, V2> pollLastEntry() {
      return transformEntry(fromMap().pollLastEntry());
    }

    @Override

      Entry<K, Collection<V>> entry = sortedMap().lastEntry();
      return (entry == null) ? null : wrapEntry(entry);
    }

    @Override
    public @Nullable Entry<K, Collection<V>> pollFirstEntry() {
      return pollAsMapEntry(entrySet().iterator());
    }

    @Override
    public @Nullable Entry<K, Collection<V>> pollLastEntry() {

   * @deprecated Unsupported operation.
   */
  @CanIgnoreReturnValue
  @Deprecated
  @Override
  @DoNotCall("Always throws UnsupportedOperationException")
  public final @Nullable Entry<K, V> pollFirstEntry() {
    throw new UnsupportedOperationException();
  }

  /**
   * Guaranteed to throw an exception and leave the map unmodified.
   *
   * @throws UnsupportedOperationException always

   * @deprecated Unsupported operation.
   */
  @CanIgnoreReturnValue
  @Deprecated
  @Override
  @DoNotCall("Always throws UnsupportedOperationException")
  public final @Nullable Entry<K, V> pollFirstEntry() {
    throw new UnsupportedOperationException();
  }

  /**
   * Guaranteed to throw an exception and leave the map unmodified.
   *
   * @throws UnsupportedOperationException always

    assertThat(map.descendingKeySet()).containsExactly("two", "three", "one").inOrder();

    assertEquals(mapEntry("one", 3), map.pollFirstEntry());
    assertEquals(mapEntry("two", 3), map.pollLastEntry());
    assertEquals(1, map.size());
  }

  @GwtIncompatible // NavigableMap
  public void testAsMapNavigableReadsThrough() {